HTTP pipelining explained

HTTP pipelining is a feature of HTTP/1.1, which allows multiple HTTP requests to be sent over a single TCP connection without waiting for the corresponding responses.^[1] HTTP/1.1 requires servers to respond to pipelined requests correctly, with non-pipelined but valid responses even if server does not support HTTP pipelining. Despite this requirement, many legacy HTTP/1.1 servers do not support pipelining correctly, forcing most HTTP clients to not use HTTP pipelining.

The technique was superseded by multiplexing via HTTP/2,^[2] which is supported by most modern browsers.^[3]

In HTTP/3, multiplexing is accomplished via QUIC which replaces TCP. This further reduces loading time, as there is no head-of-line blocking even if some packets are lost.

Motivation and limitations

The pipelining of requests results in a dramatic improvement^[4] in the loading times of HTML pages, especially over high latency connections such as satellite Internet connections. The speedup is less apparent on broadband connections, as the limitation of HTTP 1.1 still applies: the server must send its responses in the same order that the requests were received—so the entire connection remains first-in-first-out and HOL blocking can occur.

The asynchronous operations of HTTP/2 and SPDY are solution for this.^[5] By 2017 most browsers supported HTTP/2 by default which uses multiplexing instead.

Non-idempotent requests such as POST should not be pipelined.^[6] Read requests like GET and HEAD can always be pipelined. A sequence of other idempotent requests like PUT and DELETE can be pipelined or not depending on whether requests in the sequence depend on the effect of others.^[1]

HTTP pipelining requires both the client and the server to support it. HTTP/1.1 conforming servers are required to produce valid responses to pipelined requests, but may not actually process requests concurrently.^[7]

Most pipelining problems happen in HTTP intermediate nodes (hop-by-hop), i.e. in proxy servers, especially in transparent proxy servers (if one of them along the HTTP chain does not handle pipelined requests properly then nothing works as it should).

Using pipelining with HTTP proxy servers is usually not recommended also because the HOL blocking problem may really slow down proxy server responses (as the server responses must be in the same order of the received requests).^[1]

Example: if a client sends 4 pipelined GET requests to a proxy through a single connection and the first one is not in its cache then the proxy has to forward that request to the destination web server; if the following three requests are instead found in its cache, the proxy has to wait for the web server response, then it has to send it to the client and only then it can send the three cached responses too.

If instead a client opens 4 connections to a proxy and sends 1 GET request per connection (without using pipelining) the proxy can send the three cached responses to client in parallel before the response from server is received, decreasing the overall completion time (because requests are served in parallel with no head-of-line blocking problem).^[8] The same advantage exists in HTTP/2 multiplexed streams.

Implementation status

Pipelining was introduced in HTTP/1.1 and was not present in HTTP/1.0.^[9]

There have always been complaints about browsers, proxy servers, etc. not working well when using pipelined requests / responses, up to the point that for many years (at least till 2011) software developers, engineers, web experts, etc. tried to summarize the various kind of problems they noted, to fix things and to give advices about how to deal with pipelining on the Open Web.^[10]

Implementation in web browsers

Of all the major browsers, only Opera had a fully working implementation that was enabled by default. In other browsers HTTP pipelining was disabled or not implemented.

• Internet Explorer 8 does not support pipeline requests, due to concerns regarding buggy proxies and head-of-line blocking.^[11]
• Internet Explorer 11 does not support pipelining.^[12]
• Mozilla browsers (such as Mozilla Firefox, SeaMonkey and Camino) used to support pipelining; however, it was removed in Firefox 54.^[13] When it was supported, pipelining was disabled by default to avoid issues with misbehaving servers.^[14] If pipelining was enabled by the user, Mozilla browsers used some heuristics, mostly to turn pipelining off for older Microsoft IIS servers.^[15] The removal was eventually backported to SeaMonkey.
• Konqueror 2.0 supports pipelining, but it is disabled by default.^[16]
• Google Chrome previously supported pipelining, but it has been disabled due to bugs and problems with poorly behaving servers.^[17]
• Pale Moon (web browser) supports pipelining, and is enabled by default.^[18]

Implementation in web proxy servers

Most HTTP proxies do not pipeline outgoing requests.^[19]

Some HTTP proxies, including transparent HTTP proxies, may manage pipelined requests very badly (e.g. by mixing up the order of pipelined responses).^[20]

Some versions of the Squid web proxy will pipeline up to two outgoing requests. This functionality has been disabled by default and needs to be manually enabled for "bandwidth management and access logging reasons".^[21] Squid supports multiple requests from clients.

The Polipo proxy pipelines outgoing requests.^[22]

Tempesta FW, an open source application delivery controller,^[23] also pipelines requests to backend servers.^[24]

Other implementations

The libwww library made by the World Wide Web Consortium (W3C), supports pipelining since version 5.1 released at 18 February 1997.^[25]

Other application development libraries that support HTTP pipelining include:

• Perl modules providing client support for HTTP pipelining are HTTP::Async and the LWPng (libwww-perl New Generation) library.^[26]
• The Microsoft .NET Framework 3.5 supports HTTP pipelining in the module System.Net.HttpWebRequest.^[27]
• Qt class QNetworkRequest, introduced in 4.4.^[28]

Some other applications currently exploiting pipelining are:

• IceBreak application server since BUILD389
• phttpget from FreeBSD (a minimalist pipelined HTTP client)^[29]
• libcurl previously had limited support for pipelining using the CURLMOPT_PIPELINING option,^[30] but this support was removed in version 7.65.0^[31]
• portsnap (a FreeBSD ports tree distribution system)
• Advanced Packaging Tool (APT) supports pipelining.
• Subversion (SVN) has optional support for HTTP pipelining with the serf WebDAV access module (the default module, neon, does not have pipelining support).^{[32] [33]}
• Microsoft Message Queuing on Windows Server 2003 utilises pipelining on HTTP by default, and can be configured to use it on HTTPS.^[34]
• IBM CICS 3.1 supports HTTP pipelining within its client.^[35]

Testing tools which support HTTP pipelining include:

• httperf^[36]
• Nmap supports pipelining requests with the Nmap Scripting Engine (NSE)^[37]

See also

• HTTP persistent connection
• WebSocket
• SPDY

External links

• RFC 7230 Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing. 2014 . ietf.org. 10.17487/RFC7230 . 2014-07-24. Fielding . Reschke . R . J . Fielding . R. . Reschke . J. . free .
• HTTP/1.1 Pipelining FAQ at mozilla.org
• "Network Performance Effects of HTTP/1.1, CSS1, and PNG" at w3.org
• "Optimizing Page Load Times" article
• phttpget
• serf C library

Notes and References

1. Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing: Pipelining. 2014 . ietf.org. 10.17487/RFC7230 . 2014-07-24. Fielding . Reschke . R. . J. . Fielding . R. . Reschke . J. . free .
2. Web site: Revision 1330814 Connection management in HTTP/1.x MDN. MDN Web Docs. en-US. 2018-03-19. 2018-03-19. https://web.archive.org/web/20180319213834/https://developer.mozilla.org/en-US/docs/Web/HTTP/Connection_management_in_HTTP_1.x$revision/1330814. dead.
3. Web site: HTTP2 browser support. March 9, 2017.
4. Web site: Network Performance Effects of HTTP/1.1, CSS1, and PNG. World Wide Web Consortium. 14 January 2010. 24 June 1997. Henrik Frystyk. Nielsen. Henrik Frystyk Nielsen. Jim. Gettys. Jim Gettys. Anselm. Baird-Smith. Eric. Prud'hommeaux. Håkon Wium. Lie. Håkon Wium Lie. Chris. Lilley. Chris Lilley (computer scientist).
5. Web site: Reducing HTTP latency with SPDY. Nathan. Willis. 18 November 2009. LWN.net.
6. Web site: Connections. w3.org.
7. Web site: HTTP/1.1 Pipelining FAQ'.
8. Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing: Concurrency. 2014 . ietf.org. 10.17487/RFC7230 . 2014-07-24. Fielding . Reschke . R. . J. . Fielding . R. . Reschke . J. . free .
9. Web site: Key Differences between HTTP/1.0 and HTTP/1.1 . 2016-04-16 . https://web.archive.org/web/20160424204340/http://www8.org/w8-papers/5c-protocols/key/key.html . 2016-04-24 . dead .
10. Web site: Making HTTP Pipelining Usable on the Open Web. March 14, 2011. Mark Nottingham. October 16, 2021.
11. Web site: Wayback link of 'Windows Internet Explorer 8 Expert Zone Chat (August 14, 2008)'. August 14, 2008. Microsoft. May 10, 2012. https://web.archive.org/web/20101204053757/http://www.microsoft.com/windowsxp/expertzone/chats/transcripts/08_0814_ez_ie8.mspx. December 4, 2010. dead.
12. News: Internet Explorer and Connection Limits. IEBlog. 2016-11-14.
13. https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/54 Firefox 54 Release Notes
14. Web site: Bug 264354: Enable HTTP pipelining by default. Mozilla. September 16, 2011.
15. Web site: Source code – nsHttpConnection.cpp. Mozilla. Firefox source code. May 7, 2010. December 5, 2010.
16. Book: Internet Communication: Protocols and related subjects. Emir Arian. 2021-10-16. en.
17. https://www.chromium.org/developers/design-documents/network-stack/http-pipelining HTTP Pipelining - The Chromium Projects
18. Web site: HTTP/1 Pipelining support has been removed in Firefox 54 - Pale Moon forum. forum.palemoon.org. en-us. 2018-06-07.
19. Web site: The State of Proxy Caching. June 20, 2007. Mark Nottingham. May 16, 2009.
20. Web site: What proxies must do. July 11, 2011. Mark Nottingham. October 16, 2021.
21. Web site: squid : pipeline_prefetch configuration directive. November 9, 2009. Squid. December 1, 2009.
22. Web site: Polipo — a caching web proxy. September 18, 2009. Juliusz Chroboczek. November 12, 2009.
23. Web site: Tempesta FW — a Linux Application Delivery Controller. GitHub. March 29, 2018.
24. Web site: Servers: Tempesta's side - tempesta-tech/tempesta Wiki. August 1, 2017. Tempesta Technologies INC. March 29, 2018.
25. Web site: Kahan. José. Change History of libwww. World Wide Web Consortium. August 3, 2010. June 7, 2002.
26. Web site: Using HTTP::Async for Parallel HTTP Requests (Colin Bradford) . 2010-08-03 . https://web.archive.org/web/20120310042434/http://miltonkeynes.pm.org/talks/2010/02/colin_bradford_http_async.pdf . 2012-03-10 . dead .
27. http://blogs.msdn.com/b/mflasko/archive/2006/07/17/669293.aspx System.Net.HttpWebRequest & pipelining
28. http://doc.qt.nokia.com/4.6/qnetworkrequest.html QNetworkRequest Class Reference
29. http://www.daemonology.net/phttpget/ Pipelined HTTP GET utility
30. http://curl.haxx.se/dev/readme-pipelining.html Curl pipelining explanation
31. https://daniel.haxx.se/blog/2019/04/06/curl-says-bye-bye-to-pipelining/ Curl pipelining removal announcement
32. Book: Version Control with Subversion. C. Michael Pilato . Ben Collins-Sussman . Brian W. Fitzpatrick . 238. O'Reilly Media. 2008. 978-0-596-51033-6.
33. Web site: Subversion: Powerful New Toys. Justin R. Erenkrantz. 2007.
34. Web site: HTTP/HTTPS messages. Microsoft TechNet. January 21, 2005.
35. http://publib.boulder.ibm.com/infocenter/cicsts/v3r1/index.jsp?topic=/com.ibm.cics.ts31.doc/dfhtl/topics/dfhtl_cwspipelining.htm How CICS Web support handles pipelining
36. Web site: HTTP Website . 2010-10-01 . https://web.archive.org/web/20120608143409/http://www.hpl.hp.com/research/linux/httperf/ . 2012-06-08 . dead .
37. Web site: Library http . 2024-11-12.